The subject matter disclosed herein relates to control of electric motors and, in particular, to estimating the position of a rotor in a synchronous motor without a position feedback device.
Synchronous motors are electric motors having a magnetic field established in both the rotor and the stator of the motor. The magnetic field in the rotor is typically generated by passing a DC current through slip rings to the winding of the rotor. Alternately, permanent magnets may be mounted on or in the rotor to provide a continuous magnetic field in the rotor. The magnetic field in the stator is typically generated by applying an AC voltage to the winding of the stator at a frequency that matches the frequency of rotation of the magnetic field in the rotor. As a result, knowledge of the position of the rotor is important for high performance control of the synchronous motor.
It is known that high performance control of synchronous motors can be achieved utilizing a feedback device which provides rotor position information to the motor controller. The feedback device is typically an encoder mechanically coupled to the shaft of the rotor and further including a connector or lead wires for electrical connection of the encoder to the motor controller. The mechanical coupling to the shaft of the rotor and the electrical connection to the motor controller result in additional material and installation expense. The encoder further creates an additional source of failure and subsequent maintenance expense within the system. It is, therefore, desirable to provide high performance control of a synchronous motor without using a feedback device.
The most common method for attempting sensorless control of synchronous motors is to provide a position observer that models the operation of the synchronous machine to output an estimate of the rotor position. However, position observers vary for different types of synchronous machines. The wide variety of synchronous machines, therefore, presents a significant challenge for developing a universal position observer capable of providing an accurate rotor position estimate for each type of synchronous machine.
The most common position observer is based on a back EMF model of the synchronous motor. However, back EMF models for many types of synchronous motors are subject to a fundamental limitation: these motor models are speed dependent. A motor drive typically samples motor voltages and currents at a frequency significantly greater than the electrical frequency applied to the stator. The applied stator frequency corresponds to the rotational speed of the motor. If the motor is rotating at a high speed, the back EMF is high and the voltage and current values applied to the stator will be significantly affected by the back EMF. However, if the motor is rotating at a low speed, the back EMF is small and it is possible that the voltage and current values applied to the stator may not be affected by the back EMF. The drive may erroneously conclude that the motor is operating at zero speed when, in fact, the motor is rotating at a low speed. Consequently, back EMF observers can perform well at high speed but are not suited for operation at low or zero speed.
Position observers based on other motor models have been developed in an attempt to address this limitation. These observers may not be suitable for all types of synchronous motors. For example, the observer may require that the d axis and q axis inductance values be equal, making the observer suitable only for non-salient pole synchronous machines. Other models make assumptions that are not valid for all operating conditions. For example, one model assumes that the estimated speed error and the rate of change of the extended EMF are both zero between samples. While such assumptions may be valid under constant operating conditions, the assumptions are not always valid during transient conditions, such as start-up or when the load on the motor changes. Consequently, these other position observers have not provided acceptable performance over a broad range of motors and operating conditions.
Still one other method has been attempted to achieve sensorless control of synchronous machines: signal injection. Signal injection involves injecting an electrical signal into the stator at a frequency substantially higher than the fundamental frequency of the motor. Signal injection techniques typically have poor dynamic response. As a result, signal injection predicts rotor position best at low or zero speed and is not suited for operation at high speed.